Generally, a gear crank uses one front chain gear, but, as is well-known, may use two or more front chain gears having a different number of teeth. A derailleur switches a driving chain to a desired chain gear, so that the driving force from pedalling is transmitted to a rear chain gear at a rear wheel hub of the bicycle at a given gear ratio for changing speed.
The front chain gear incorporated with a crank means is formed of a lightweight material such as aluminum. It is made of lightweight material because it has more teeth than those of the rear chain gear and is inherently larger in diameter and in weight than the rear chain gear so that the lightweight material serves to reduce overall weight.
Such lightweight material is less rigid and has a problem with wear-resistance. Therefore, when two or more chain gears formed of the lightweight material are assembled into a multi-stage chain gear and incorporated with the crank means, teeth of the gears wear out or are broken while changing speed, making durability of the teeth a problem.
When the multi-stage chain gear switches the chain from a smaller diameter chain gear to a larger diameter one, a tooth thereof adjacent to a chain catching tooth and rearward thereof in the rotation direction of the larger diameter chain gear, contacts with the chain travelling slantwise. The chain, when switched from the larger diameter chain gear to the smaller diameter one, contacts in a twisting manner with a tooth adjacent to a chain disengaging tooth of the larger diameter chain gear and forward of the disengaging tooth in the direction of rotation. Hence, these adjacent teeth are subjected to a considerable load resulting in wearing or breaking of the teeth.
The above problem may be solved by using chain gears formed of a material, such as iron, superior in rigidity and wear-resistance to thereby improve its durability. Such chain gears, however, become not only larger in weight but also expensive to produce and are still defective, not withstanding having the advantage of the use of lightweight material.
The inventor has proposed, in U.S. Pat. No. 54,910, a chain gear which is formed of a lightweight material and omits some teeth, so that a toothed member formed of a material, such as iron, superior in rigidity and wear-resistance, is fitted into the teeth-omitted portion. The teeth-omitted portion is provided not for all the teeth but for some teeth likely to catch the chain when switched.
The aforesaid teeth likely to catch the chain, are located at a position coincident with the engaging point of the chain where the chain engages the teeth of the larger diameter chain gear when switched from the smaller diameter chain gear to the larger diameter one. The engaging position is located at a distance from the point where the chain begins to disengage from the smaller diameter gear equal to an integer multiple of the chain pitch. In this instance, the positional relationship between teeth of both the chain gears is changed due to the different number of teeth of each, whereby the point which engages with the larger diameter chain gear when the chain is moved thereto may or may not engage the tips of teeth of the larger diameter chain gear.
The chain has many opportunities of being switched by teeth of the toothed member located at the position where the teeth easily engages the chain, but the chain is not assured of engaging the teeth of the toothed member. Hence, the chain may engage with teeth integral with the chain gear body formed from a lightweight material. The problem of wearing or breakdown of the teeth is not therefore solved.